1. Field of the Invention
The invention relates to a circuit arrangement for supplying a periodic, substantially parabolic signal to an output terminal, which arrangement comprises a sawtooth generator for supplying a periodic sawtooth signal having a period comprising a trace time and a retrace time, a parabolic generator having an input for receiving the sawtooth signal and an output for supplying a substantially parabolic signal to the output terminal of the circuit arrangement during the trace time, and an arcuate signal generator for supplying an arcuate signal to the output terminal during the retrace time of the sawtooth signal, while at the start of the retrace time, the arcuate signal and its derivative with respect to time are given values which are substantially equal to corresponding values of the substantially parabolic signal at the end of the trace time. An arcuate signal is herein understood to mean a signal having the shape of an arch, for example, half a period of a sine wave or a part of a parabola.
2. Description of the Related Art
A circuit arrangement of this type is known from European Patent Application no. 0,273,497 (PHN 11.964) and is used, for example, for generating a control signal of an east-west modulator in a picture display device and/or for the dynamic focusing in such an arrangement. This control signal may have the field frequency (50 Hz in accordance with the European standard) or the line frequency (15.625 kHz in accordance with the said standard) or any other suitable frequency.
To prevent ringing effects of resonant circuits in the east-west modulator or in the dynamic focusing circuit to a maximum possible extent, the known circuit arrangement ensures that the arcuate signal and the parabolic signal and their derivatives continuously merge. In the known circuit arrangement, this is achieved in that the arcuate signal and its derivative with respect to time are given values at the start of the retrace time corresponding to the values of the parabolic signal and its derivative at the end of the trace time. Furthermore, the natural frequency of the arcuate signal is chosen to be such that the arcuate signal and its derivative assume values at the end of the retrace time corresponding to the values of the parabolic signal at the start of the trace time.
A drawback of the known circuit arrangement is that it is not very well possible to vary the duration of the retrace time because then the continuity between the arcuate signal and the parabolic signal at the end of the retrace time is lost. It is often desirable that the retrace time should be variable or adjustable, for example, in multi-standard picture display devices, or that the centering of the displayed picture should be adjustable.
In the generally known method of centering the sawtooth signal, a DC component is added so that the center of the picture information is displayed in the center of the display screen. If this offset sawtooth signal is squared for the purpose of east-west correction, a parabola is produced with the start and the end not being at the same level. This is undesirable because a symmetrical east-west correction is necessary. The trace time is therefore preferably varied so that the picture information is also offset on the display screen. However, this produces an opposite variation of the retrace time.